AISC-341-16 and Seismic Design Manual, RBS SMF Query

[LetsTalkEng]

I am in the process of designing a 3 story steel structure which will be seismic design category D. It has considerable architectural limitations and we are trying to make use of a Special Moment Frame (SMF) system. For simplicity sake, I have gridlines A, B, and C on each level. The columns will be exposed at each level in some degree and so we have been asked to maintain continuity of column sizes. On the bottom 2 floors we have SMF columns at A,B and C. At the upper floor we only have SMF columns at A & C. The span between A and C becomes onerous given the roof assembly thickness and the resulting increase in beam size does not work for Strong-Column Weak-Beam requirements outlined in Seismic design manual for RBS SMFs. There is a narrow wall on the upper floor at grid B that would allow for a gravity only column to exist, and we could then maintain the same beam sizes as below and we then meet Strong-Column Weak-Beam requirements again.

My question is:

Is an intermediate gravity only support of a SMF beam allowed? ie the beam is continuous over the intermediate column with a pinned support, only moment connected to SMF columns at grids A & C.

My first impression is no, as this changes the bending behavior and stress profile of the SMF beam which already needs to meet pre-qualified conditions in the first place. I just can't find a resource the explicitly says this is not a feasible solution (AISC Clause somewhere?). Perhaps if we can prove that we can achieve sufficient rotation above the interior column that still allows the plastic hing to develop at the SMF Beam-Column connection it could be validated, but I'm not sure this project has the budget available for that.

Thanks to everyone in advance.

 

[Deker]

I don't believe there is anything in the code that would prohibit SMF beams with moment connections at one end only. Take a look at the prequalification variables listed in AISC 341-16 K1.4a for some insight. In terms of seismic behavior, your proposed solution will theoretically behave identically to a frame without the intermediate column. As long as your frame satisfies drift requirements, I don't see any issues.

That said, if I were able to add an intermediate column my preference would be to use a compact wide flange that could be detailed as part of the frame. Keep in mind that strong-column weak-beam is not usually required at the roof level since there aren't significant concerns for soft/weak stories at the upper level (AISC 341-16 E3.4a(a)).

 

[KootK]

Similar to Deker, I think this is doable. Additionally:

1) I think that you're plastic mechanism in the upper level frame will need to be tailored to suit the new reality. You're gravity column probably ought to be designed for an axial load commensurate with plastic hinge development etc.

2) Conceptually, this isn't too much different from having a point load in the middle of an SMF beam which is something that is also done.

Rationally, it's quite acceptable to not satisfy strong column weak beam at the roof level. At the roof level, it's pretty much irrelevant whether you plastic hinges form in your beams or your column as that doesn't create a single story mechanism.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

...as far as mechanisms, go. If shakedown's an issue, you might want to have your hinges form in the beams.

Dik